American Association of Variable Star Observers (AAVSO)
Wed, 07/03/2013 - 20:43

Hello, I am new to VPHOT and I have a question about setting up my telescope's information. I am trying to upload images for the first time and it says I am required to input information about my telescope, but I am unsure what some of it means. I am currently using a DSLR camera (a canon rebel t1i) and I am not sure what information to use to fill out the "gain" and "linearity limit" sections. I have tried researching these and I can only find things regarding CCD cameras. Can anyone tell me where to find this information for my camera? I have tried looking at the cameras specs in the manual and cannot find it there. Any help would be greatly appreciated, Thanks so much, Talia

American Association of Variable Star Observers (AAVSO)
DSLR spec's

Hello Talia, 

I am not user of VPHOT and don't know its needs. I am making photometry with DSLR for years and usually characterize them using my own software and sometime IRIS. Regretfully the manufacturers don't provide any such technical data for their cameras. 

The Rebel T1i is also known here in EU as the 500D. It's a CMOS sensor DSLR and there is no issue of linearity with this technology (VPHOT is obviously made for CCD for which that issue could exist). The only limit is the full capacity: saturation of the sensor or clipping of the A-to-D-Converter depending ISO. For the 500D it is about 25000 electrons at ISO 100 and then it decreases inverse proportionally to the ISO. I would not recommand to get above ISO 400 ( about 6000 electrons) in most cases (specific cases could be ok up to ISO 1600)

Then, what the astro CCD guys call "gain" is usually NOT the true gain of the read amplifier as an electronic guy would understand it (like me: normally Vout/Vin instead the inverse ! ) but the calibration factor of the ADC in electron per ADU. For the 500D (APS-C 15 Mp and 14 bits) it should be about 1.8 e/ADU at ISO 100 and then proportional to (100 / ISO xxxx ), at ISO 400 this is 0.45 e/ADU. 

There is a procedure to measure that calibration factor based on the measurment of the shot noise in a flat at, let say 2/3 range, the read noise shall be subtracted (quadratically) from the total noise and then the calibration factor comes form the flat level (ADU) and the sigma of the shot noise. All tools needed to do it are in IRIS and the procedure is in:

Clear Skies !